Fluorene-based asymmetric bipolar universal hosts for white organic light emitting devices

Article abstract of DOI:10.1002/adfm.201202889

Two new bipolar host molecules composed of hole-transporting carbazole and electron-transporting cyano (CzFCN) or oxadiazole (CzFOxa)-substituted fluorenes are synthesized and characterized. The non-conjugated connections, via an sp³-hybridized carbon, effectively block the electronic interactions between electron-donating and -accepting moieties, giving CzFCN and CzFOxa bipolar charge transport features with balanced mobilities (10^-5 to 10^-6 cm² V^-1 s^-1). The meta-meta configuration of the fluorene-based acceptors allows the bipolar hosts to retain relatively high triplet energies [E_T = 2.70 eV (CzFOxa) and 2. 86 eV (CzFCN)], which are sufficiently high for hosting blue phosphor. Using a common device structure - ITO/PEDOT:PSS/DTAF/TCTA/host:10% dopants (from blue to red)/DPPS/LiF/Al - highly efficient electrophosphorescent devices are successfully achieved. CzFCN-based devices demonstrate better performance characteristics, with maximum η_ext of 15.1%, 17.9%, 17.4%, 18%, and 20% for blue (FIrpic), green [(PPy)₂Ir(acac)], yellowish-green [m-(Tpm)₂Ir(acac)], yellow [(Bt)₂Ir(acac)], and red [Os(bpftz)₂(PPhMe₂)₂, OS1], respectively. In addition, combining yellowish-green m-(Tpm)₂Ir(acac) with a blue emitter (FIrpic) and a red emitter (OS1) within a single emitting layer hosted by bipolar CzFCN, three-color electrophosphorescent WOLEDs with high efficiencies (17.3%, 33.4 cd A^-1, 30 lm W ^-1), high color stability, and high color-rendering index (CRI) of 89.7 can also be realized. Copyright

Full text of DOI:10.1002/adfm.201202889

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Fluorene-Based Asymmetric Bipolar Universal Hosts
for White Organic Light Emitting Devices
Ejabul Mondal, Wen-Yi Hung,* Hung-Chi Dai, and Ken-Tsung Wong*
annihilation, is essential. Therefore, the
host–guest strategy has been commonly
used in PhOLEDs with triplet emitters
(guest) homogeneously dispersed within
suitable host materials. In this regard,
the developments of new host materials
are as important as the design of efficient
triplet emitters with promising characters
for realizing the practical applications
of PhOLEDs. The host materials play an
important role for charge transport and
electron/hole balance as well as confine-
ment of emissive excitons in the emissive
layer, which are important factors respon-
sible for high-efficiency PhOLEDs.[3] One
promising molecular design strategy of
host materials to achieve high and bal-
anced electric flux is to incorporate elec-
tron-donating (D) and electron-accepting
(A) components into a single bipolar mol-
ecule, thereby facilitating injection and
transport of holes and electrons into the
emitting layer.[4] However, the bipolar host
material hybridizing the individual charac-
teristics of D and A moieties should avoid
efficient intramolecular charge transfer to
give triplet energy higher than the phos-
Two new bipolar host molecules composed of hole-transporting carbazole
and electron-transporting cyano (CzFCN) or oxadiazole (CzFOxa)-substituted
fluorenes are synthesized and characterized. The non-conjugated connec-
tions, via an sp3-hybridized carbon, effectively block the electronic interactions
between electron-donating and -accepting moieties, giving CzFCN and CzFOxa
bipolar charge transport features with balanced mobilities (10−5 to 10−6 cm2 V−1 s−1 ).
The meta–meta configuration of the fluorene-based acceptors allows the
bipolar hosts to retain relatively high triplet energies [ET = 2.70 eV (CzFOxa)
and 2. 86 eV (CzFCN)], which are sufficiently high for hosting blue phosphor.
Using a common device structure – ITO/PEDOT:PSS/DTAF/TCTA/host:10%
dopants (from blue to red)/DPPS/LiF/Al – highly efficient electrophosphores-
cent devices are successfully achieved. CzFCN-based devices demonstrate
better performance characteristics, with maximum ηext of 15.1%, 17.9%,
17.4%, 18%, and 20% for blue (FIrpic), green [(PPy)2Ir(acac)], yellowish-green
[m-(Tpm)2Ir(acac)], yellow [(Bt)2Ir(acac)], and red [Os(bpftz)2(PPhMe2)2, OS1],
respectively. In addition, combining yellowish-green m-(Tpm)2Ir(acac) with
a blue emitter (FIrpic) and a red emitter (OS1) within a single emitting layer
hosted by bipolar CzFCN, three-color electrophosphorescent WOLEDs with
high efficiencies (17.3%, 33.4 cd A−1 , 30 lm W −1 ), high color stability, and high
color-rendering index (CRI) of 89.7 can also be realized.
phor to prevent reverse energy transfer from guest to host.[5]
This becomes more challenging for host materials used in
blue phosphorescent emitter. To preserve high triplet energy,
the interruption of the π-conjugation between D and A compo-
nents in a bipolar molecule by using highly-twisted or saturated
spacers is crucial.[4] For cost-effective fabrication of full-color
PhOLED displays, it is highly desired that the host material is
concurrently suitable for red (R), green (G), and blue (B) phos-
phors with the same device structure to give high electrolu-
minescence (EL) efficiencies for individual emitting colors. In
addition, by combining two or three appropriate efficient phos-
phorescent emitters, all-phosphor white organic light-emitting
diodes (WOLEDs) using a universal host material can also be
reasonably achieved. Therefore, efforts are ongoing for the
development of universal bipolar host materials with high tri-
plet energy (≥2.7 eV) suitable for various phosphors in high-
efficiency PhOLEDs. Nevertheless, the reported host materials
concurrently suitable for RGB phosphors are relatively lim-
ited.[6] For example, Cheng recently reported a universal bipolar
host material bis-[4-(N-carbazolyl)phenyl]phenylphosphine
oxide (BCPO) containing a phosphine oxide as acceptor and two
carbazole groups as donor exhibiting promising properties,[6a]
which can be applied as an efficient host for blue, green, and red
1. Introduction
Phosphorescent organic light-emitting devices (PhOLEDs)
are of extensive research interest in academia and industry
because of their potential applications in full-color flat-panel
displays and thin-film lighting.[1] Due to the strong spin-orbit
coupling, PhOLEDs with heavy metal-centered phosphors can
efficiently harvest both the singlet and triplet excitons, thus
enabling a 100% internal quantum efficiency (IQE).[2] In order
to achieve unity IQE, the suppression of detrimental effects of
phosphors, such as aggregation quenching and triplet–triplet
Dr. E. Mondal, Prof. K.-T. Wong
Department of Chemistry
National Taiwan University
Taipei 10617, Taiwan (R.O.C.)
E-mail: kenwong@ntu.edu.tw
Prof. W.-Y. Hung, H.-C. Dai
Institute of Optoelectronic Sciences
National Taiwan Ocean University
Keelung 202, Taiwan (R.O.C.)
E-mail: wenhung@mail.ntou.edu.tw
DOI: 10.1002/adfm.201202889
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phosphorescent devices with extremely high external quantum
efficiencies (ηext). Recently, we reported a new bipolar host
CMesB by incorporating dimesityl borane at the C3 and C6 posi-
tions of N-phenylcarbazole.[6e] PhOLEDs using CMesB as host
doped with various emitters yield ηext of 20.7% for red, 20.0%
for green, 16.5% for blue, and 15.7% for white OLEDs at prac-
tical brightness. However, these excellent hosts usually adopted
different device structures for individual dopants for giving
optimal device performance. The reported universal bipolar
host materials suitable for various phosphors under the same
device architecture are relatively rare. One elegant example was
reported by Yang, where a bipolar host p-BISiTPA comprising
silicon-bridged triphenylamine (D) and benzimidazole (A) moi-
eties, giving a high triplet energy (ET) of 2.69 eV and bipolar
transporting features.[6b] Devices employing p-BISiTPA as host
exhibited ηext as high as 16.1% for blue, 22.7% for green, 20.5%
for orange, and 19.1% for all-phosphor (two-color) WOLEDs
under a common device structure. Recently, we reported a
universal bipolar host CNBzIm with high Tg of 180 °C and tri-
plet energy (ET = 2.71 eV), by incorporating electron-accepting
N-phenylbenzimidazole units via N-bridge connection at the C3
and C6 positions of N-phenylcarbazole.[6d] PhOLEDs employing
CNBzIm as a host doped with various phosphors under the
same device structure show excellent performance, with ηext as
high as 12.7% for blue, 17.8% for green, 19.1% for red. A two-
color, all-phosphor, and single-emitting-layer WOLEDs hosted
by CNBzIm was also achieved with maximum efficiencies of
15.7%, 35 cd A−1 , and 36.6 lm W−1 .
acceptor. An electron-withdrawing cyano group was selected as
acceptor because it can enhance both the electron affinity for
efficient electron injection and electron transport.[7] In addition,
the oxadiazole group, derivatized easily from the cyano group,
is a well-established electron-transporting (ET) functional
group used in OLEDs.[8] Obviously, direct electronic interac-
tions between donor and acceptor(s) can be efficiently blocked
with a saturated spacer (C9 of fluorene). In addition, the meta–
meta configurations of the acceptor moieties ensure limited
extension of π-conjugation for preserving high triplet energies.
Moreover, the presence of a tolyl group makes the C9 position
of fluorene a chirality center, leading the resultant bipolar mol-
ecule to be a racemic mixture, which we believe will ensure an
amorphous character and thus thin-film stability of the emitting
layer. These two novel hosts were suitable for various phospho-
rescent emitters, including FIrpic, (PPy)2Ir(acac), (Bt)2Ir(acac),
Os(bpftz)2(PPhMe2)2,and a new yellowish-green emitter bis-(m-
totylpyrimidine)(acetylacetonate)iridium(III) (m-(Tpm)2Ir(acac);
see Scheme S1 in the Supporting Information, SI) with excel-
lent device performance. PhOLEDs employing CzFCN as a host
doped with RGB phosphors under the common device struc-
ture of ITO/PEDOT:PSS (30 nm)/DTAF (20 nm)/TCTA (5 nm)/
CzFCN:10% dopant (25 nm)/DPPS (50 nm)/LiF/Al with ηext as
high as 15.1% for blue, 17.9% for green, 17.4% for yellowish-
green, 18% for yellow, and 20% for red were achieved. More
importantly,CzFCN-hosteddevicesdemonstratedremarkablesta-
bility with a low efficiency roll-off at a brightness of 1000 cd m−2 .
On the other hand, devices with CzFOxa as host using the
same device structure provided good performance with ηext
of 9.2% for blue, 14.5% for green, 16.2% for yellowish-green,
17.6% for yellow, and 19.4% for red, respectively. In addition,
by combining a new yellowish-green emitter m-(Tpm)2Ir(acac)
with a blue emitter (FIrpic) and a red emitter
In this paper, we report two bipolar host materials CzFCN
and CzFOxa (Scheme 1), in which the C9 of fluorene was
adopted as the nonconjugated bridge to connect hole-trans-
porting carbazole as donor and C3-substituted fluorenes as
(OS1), three-color, all-phosphor, and single
emitting layer WOLEDs hosted by CzFCN
and CzFOxa were obtained with three evenly
separated main RGB peaks with maximum
ηext of 17.3% and 15.7%, power efficiencies
(ηp) of 30 lm W−1 , 28 lm W−1 , respectively,
and a high color-rendering index (CRI) of up
to 89. The WOLEDs exhibited high color sta-
bilities owing to broad charge recombination
zones within the emitting layer incorporated
within these novel bipolar hosts.
2. Results and Discussion
2.1. Synthesis and Characterization
Scheme 1 illustrates our synthetic routes
towards CzFCN and CzFOxa. The reac-
tion of 3-bromofluorenone (1)[9] with the
Grignard reagent 4-bromotoluene gave the
corresponding alcohol 3-bromo-9-p-tolyl-9H-
fluoren-9-ol (2) in 93% yield. 9-Phenylcarba-
zole 3 was treated with alcohol 2 in CH2Cl2
using Eaton’s reagent[10] as the catalyst and
Scheme 1. Synthetic routes to CzFCN and CzFOxa.
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T a b le 1 . Physical properties of CzFCN and CzFOxa.
OX
RED
Host
Tg
[°C]
Td
[°C]
E1/2
E1/2
[V]
HOMO
[eV]a)
LUMO
[eV]b)
ET
[eV]
ΔEg
Abs λmax
PL λmax
[eV]c)
[nm]solid/film
243,298/243,300
285,331/285,336
[nm]solid/film
357,373/357
415/390
[V]
CzFCN
134
170
335
406
1.35
1.35
– 2.21
– 1.99
– 5.65
– 5.8
– 2.18
– 2.45
3.47
3.35
2.86
2.70
CzFOxa
a)HOMO determined using photoelectron yield spectroscopy (AC-2); b,c)LUMO is determined from the equation LUMO = HOMO + Eg, where Eg was calculated from the
absorption onset of the solid film.
condensing agent at room temperature, giving 4 in 73% isolated
ET=2.86 eV
yield after column chromatography. Bipolar host CzFCN was
obtained in 60% yield as compound 4 was reacted with CuCN
in dimethylformamide (DMF) and refluxed for 24 h at 148 °C.
Bipolar host CzFOxa was synthesized from CzFCN. CzFCN
treated with NaN3 and NH4Cl in DMF at 100 °C gave tetrazole
intermediate 5 in 90% yield, which was subsequently treating
with 4-tert-butylbenzoyl chloride in pyridine to afford CzFOxa
in 80% yield. All the intermediates and CzFCN, CzFOxa were
CzFCN
Film
Solu.
Phos.
ET=2.70 eV
CzFOxa
1
fully characterized using H and 13C NMR spectroscopy, mass
spectrometry, and elemental analysis (see the experimental
section in the SI). In addition, a new pyrimidine-based green
phosphor, m-(Tpm)2Ir(acac), was synthesized (see Scheme S1
in the SI). The photophysical and electrochemical properties of
m-(Tpm)2Ir(acac) are shown in Figures S1 and S2, respectively,
and summarized in Table S1 in SI.
300
400
500
600
Wavelength (nm)
2.2. Thermal Properties
Figure 1. Room-temperature absorption and emission (PL) spectra of
CzFCN and CzFOxa in CH2Cl2 and neat films as well as corresponding
phosphorescence (Phos) spectra recorded in EtOH at 77 K.
The morphological and thermal properties of bipolar hosts
CzFCN and CzFOxa were investigated by differential scan-
ning calorimetry (DSC) and thermogravimetric analysis (TGA;
Table 1). Host materials CzFCN and CzFOxa exhibited high
glass transition temperatures (Tg) of 134 and 170 °C, respec-
tively. In addition, no crystallization or melt was detected at
the second heating. Their decomposition temperatures were
recorded at 335 and 406 °C, respectively (Td; corresponding
to 5% weight loss). The amorphous behavior of CzFCN and
CzFOxa can be reasonably attributed to the character of racemic
mixture. The high Tg and Td values of CzFOxa as compared to
CzFCN are presumably because of higher molecular weight.
The good thermal stability with high Tg is beneficial for these
host materials to form homogeneous and amorphous films,
verified by the atomic force microscopic (AFM) analyses shown
in Figure S3 in the SI, which are crucial for successful use in
thin-film devices by vacuum deposition.
with maximum centered at λmax 357 nm. As compared to those
of CzFCN, CzFOxa exhibit red-shifted absorption (332 nm) and
emission (415 nm) spectra due to the extended π-conjugation
along with oxadiazole substitution. To gain more insight into
the photophysical properties, we studied the emission behavior
of CzFCN and CzFOxa in various solutions (1 × 10−5 M) (see
Figure S4 to S7 in the SI). The evident solvatochromic behavior
of CzFOxa indicates the occurrence of photoinduced intramo-
lecular charge transfer (ICT), leading to a polar excited state
which was sensitive to the polarity of solvents. In contrast, the
limited changes in the emission spectra of CzFCN in different
solvents reveal the ineffective ICT behavior.[11] Apparently, the
introduction of oxadiazole onto the fluorene core leads to a
lower lowest unoccupied molecular orbital (LUMO) level, giving
a good tendency for an efficient ICT between the carbazole
donor and oxadiazole-substituted acceptor. The phosphores-
cence spectra of CzFCN and CzFOxa were measured in EtOH
at 77 K and the triplet energies (ET) were estimated, from the
highest energy vibronic band of the phosphorescence spectra,
to be 2.86 and 2.70 eV, respectively. The high triplet energy of
CzFCN is comparable to previously reported host material,[12]
revealing that the introduction of a CN group at the C3 posi-
tion of the fluorene unit has a weak effect on increasing the ET
due to the meta–meta configuration. However, the introduced
oxadiazole in CzFOxa has evident influence on the resultant
2.3. Photophysical Properties
Figure 1 presents the UV-vis absorption and emission (PL)
spectra of CzFCN and CzFOxa in dilute solution (CH2Cl2)
and neat films. The photophysical data are summarized in
Table 1. The absorption peak of CzFCN around 300 nm could
be assigned to the π–π∗ transition, while weak shoulder peaks
at 326 and 350 nm are attributed to the n–π∗ transitions of car-
bazole. The emission of CzFCN showed clear vibronic feature
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time-of-flight (TOF) techniques to measure the associated car-
rier mobility.[14] The device used for TOF measurement was
prepared through vacuum deposition of the tri-layer struc-
ture as: ITO glass/CzFCN (1.76 μm) or CzFOxa (2.07 μm)/Ag
(150 nm), followed by placing them inside a vacuum chamber
for data measurement. Figure 3a–d display typical room-tem-
perature TOF transients for both materials under an applied
electric field, for which we observed typical transient photocur-
rents from both holes and electrons. Using double-logarithmic
representations (insets to Figure 3a–d), we extracted the carrier-
transit times (tT), which are required to determine the carrier
mobilities, from the intersections of the two asymptotes. We
then calculated the mobility using the formula μ = d2/VtT, where
d is the sample thickness and V is the applied voltage. Figure 4
shows the carrier mobilities of CzFCN and CzFOxa plotted as a
function of the square root of the applied electric field, as well
as the nearly universal Poole-Frenkel relationship. The electron
mobilities of CzFCN lie in the range 1.1 × 10−5 to 2.4 × 10−5 cm2
V−1 s−1 for fields varying from 5.1 × 105 to 8.0 × 105 V cm−1 ;
these are higher than the hole mobilities, which are within the
range 3.6 × 10−6 to 1.2 × 10−5 cm2 V−1 s−1 for fields varying from
4.0 × 105 to 6.8 × 105 V cm−1 . This tendency is very similar to
those of CzFOxa: the electron mobilities lie in the range 3.3 ×
10−6 to 9.3 × 10−6 cm2 V−1 s−1 for fields varying from 2.9 × 105 to
5.8 × 105 V cm−1 , and the hole mobilities in the range 1.4 × 10−6
to 4.4 × 10−6 cm2 V−1 s−1 for fields varying from 1.9 × 105 to
5.8 × 105 V cm−1 . These results indicate that the CzFCN and
CzFOxa show bipolar charge-transport properties with balance
electron/hole mobilities. However, the hole- and electron-mobil-
ities of CzFCN are ca. two-times greater than those of CzFOxa.
This can be attributed to the attachment of less bulky electron-
withdrawing cyano onto fluorene results in close molecular
80
40
0
CzFCN
CzFOxa
-40
-80
2
1
0
-1
-2
-3
Potential (V vs Ag / AgCl)
Figure 2. Cyclic voltammogram of CzFCN and CzFOxa. The measure-
ment of oxidation potentials were performed in CH2Cl2 with 0.1 M of
nBu4NPF6 as a supporting electrolyte, and the reduction CV were per-
formed in tetrahydrofuran (THF)/DMF with 0.1 M of nBu4NClO4 as a sup-
porting electrolyte.
bipolar molecule. Nevertheless, the triplet energies of CzFCN
and CzFOxa are sufficiently high to serve as host materials for
various RGB triplet emitters.
2.4. Electrochemical Properties
Cyclic voltammetry (CV) was performed to study the elec-
trochemical properties of bipolar host materials CzFCN and
CzFOxa (Figure 2). The electrochemical data of CzFCN and
CzFOxa are summarized in Table 1. The observed irreversible
oxidation behaviors of CzFCN and CzFOxa at 1.35 V (vs. Ag/
AgCl) are typical for C3, C6–unprotected 9-phenylcarbazole,
while the new low potential peaks at 1.01 V
(vs. Ag/AgCl) indicate the dimerization of
(a)1000
(c) 300
carbazole.[13] On the other hand, both CzFCN
CzFOxa(h)
CzFCN(h)
800
600
400
200
0
and CzFOxa exhibited a well-defined quasi-
reversible reduction process at –2.21 and
–1.99 V (vs. Ag/AgCl), respectively, arising
from the cyano- and oxadiazole-substituted
fluorene. The elctrochemical properties of
CzFCN and CzFOxa clearly indicated their
bipolarity, revealing the potential hole/elec-
tron transport properties in OLED devices.
By using a Ricken AC-2 photoemission spec-
trometer, we determined the highest occupied
molecular orbital (HOMO) energy level of
CzFCN and CzFOxa to be –5.65 and –5.80 eV,
respectively. From the equation LUMO =
HOMO + ΔEg, where ΔEg is the optical
bandgap determined from the absorption
threshold, we estimated the LUMO energy
level of CzFCN and CzFOxa to be –2.18 and
–2.45 eV, respectively.
250
tT
100
100
tT
200
150
100
50
10
1
10
1
0.1
10
100
Time (μsec)
10
100 1000
Time (μsec)
0
0
50
100
150
200
0
200 400 600 800 1000 1200
Time (μsec)
Time (μsec)
150
120
90
60
30
0
(d)250
(b)
CzFCN(e)
CzFOxa(e)
100
50
40
200
tT
30
20
tT
150
100
50
10
1
10
10
100
Time (μsec)
1000
1
10
Time (μsec)
100
0
0
30 60 90 120 150 180
0
200 400 600 800 1000
Time (μsec)
Time (μsec)
2.5. Charge-Carrier Mobility
Figure 3. Typical transient photocurrent signals for CzFCN (1.76 μm thick) at E = 5.7 ×
105 V cm−1 and CzFOxa (2.07 μm thick) at E = 3.9 × 105 V cm−1 :a,c) holes, and, b,d) electrons.
Insets are the corresponding double logarithmic plots.
In order to confirm the bipolar transporting
properties of CzFCN and CzFOxa, we used
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yellow-green, yellow, and red electrophosphorescent devices
using
CzFCN
Electron
Iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C2´]
picolinate (FIrpic),[15] bis(2-phenylpyridinato)iridium(III) acety-
lacetonate [(PPy)2Ir(acac)],[16] bis(m-tolylpyrimidine)iridium(III)
acetylacetonate [m-(Tpm)2Ir(acac)], bis(2-phenylbenzothiazolato)
(acetylacetonate)iridium(III) [(Bt)2Ir(acac)],[17] and osmium(II)
bis[3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolate]
dimethylphenylphosphine [Os(bpftz)2(PPhMe2)2, OS1][18] as
dopants (Scheme 2), respectively, in the structure: ITO/poly
Hole
CzFOxa
1E-5
Electron
Hole
.
1E-6
ethylenedioxythiophene:polystyrenesulfonate
(PEDOT:PSS,
30
20
nm)/9,9-di[4-(di-p-tolyl)aminophenyl]fluorene
(DTAF,
(TCTA,
400
500
600
700
800
900
nm)[19]/4,4′,4″-tri(N-carbazolyl)triphenylamine
E1/2 (V/cm)1/2
5 nm)[20]/host:dopant (10 wt-%, 25 nm)/diphenyl-bis[4-(pyridin-
3-yl)phenyl]silane (DPPS, 50 nm)[21]/LiF (0.5 nm)/Al (100 nm).
To maximize PhOLED efficiency, a high ET hole-transporter
(HTL) and electron-transporter (ETL) are also imperative to pre-
vent any possible luminescence quenching by the carrier-trans-
porting layers and to confine the triplet excitons in the EML. As
shown in our previous report, DTAF with a high triplet level (ET
= 2.87 eV) and wide bandgap (Eg = 3.47 eV) can efficiently con-
fine the triplet energy of phosphors, is selected as the HTL.[19]
TCTA, (ET: 2.76 eV; HOMO/LUMO: 5.7/2.4 eV) is selected to act
as both triplet exciton blocker and HTL to stepwise reduce the
hole injection barrier from DTAF to EML. To further confine
Figure 4. Electron and hole mobilities versus E1/2 for CzFCN and
CzFOxa.
packings than the bulky oxadiazole in CzFOxa, which induce
easier carrier hopping among the adjacent molecules.
2.6. Monochromatic Devices
To evaluate the applications of bipolar molecules CzFCN
and CzFOxa as host materials, we fabricated blue, green,
HTL
EML
ETL
1.75eV
2.18eV
2.4eV 2.45eV
2.5eV
Cathode
DTAF
20 nm
CzFOxa
CzFCN
DPPS
50 nm
TCTA
5 nm
25 nm
Anode
5.22eV
5.65eV
5.7eV
5.8eV
dopants
6.5eV
Scheme 2. Molecular structures used in this study and an energy level diagram of the device.
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T a b le 2 . EL performance of devices.
Von
[V]b)
Lmax
[cd m−2
Imax
[mA cm−2
CIE
[x,y]
ηext
[%]
ηc
ηp
ηext
[cd A−1
]
[lm W−1
]
[V,%]c)
]
]
B1a)
G1
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
3.0
2.5
3.0
38 200 (14 V)
150 600 (14 V)
103 000 (15 V)
80 600 (16.5 V)
26 200 (16.5 V)
54 300 (14.5 V)
35 300 (13.5 V)
160 000 (14.5 V)
126 600 (14.5 V)
99 600 (15 V)
520
920
850
690
650
760
800
1170
920
1020
890
790
720
1090
630
15.1
17.9
17.4
18
31.6
64.7
60.4
44.1
21.8
33.4
22
18.5
60
7.8, 15.1
6.0, 17.9
6.5, 17.4
8.5, 17.1
8.5, 17.3
8.0, 14.2
6.6, 8.9
0.17,0.35
0.35,0.62
0.43,0.56
0.53,0.47
0.65,0.35
0.45,0.43
0.18,0.39
0.35,0.62
0.42,0.56
0.52,0.48
0.65,0.34
0.46,0.43
0.16,0.33
0.35,0.61
0.64,0.35
YG1
Y1
40
34
R1
20
20
W1
B2
17.3
9.2
30
19.2
50
G2
14.5
16.2
17.6
19.4
15.7
17.7
14.0
15.7
53.2
53.6
45.4
21.3
31.2
35.6
48.2
15.0
5.0, 14.4
6.0, 14.5
6.2, 13.3
6.5, 16.5
7.5, 12.3
7.1, 17.3
5.7, 11.5
7.6, 15.7
YG2
Y2
53.2
47.5
22.3
28
R2
32 300 (13.5 V)
47 000 (13 V)
W2
B3d)
Y3d)
R3d)
45 300 (15.5 V)
162 300 (14 V)
24 800 (14.5 V)
22.5
22.3
7.6
a)The notation 1 and 2 indicate the devices fabricated with host materials of CzFCN and CzFOxa, respectively; b)Turn-on voltage at which emission became detectable; c)ηext
;
and driving voltage of device at 1000 cd m−2 d)For comparison, references cell with emitting layer of mCP:FIrpic (B3), CBP:(PPy)2Ir(acac) (G3), and CBP:OS1 (R3) were
fabricated with the same device structure.
the holes or generated excitons within the emissive region,
DPPS (ET: 2.7 eV; HOMO/LUMO: 6.5/2.5 eV) is selected as the
ETL to block the excitons within the EML. PEDOT:PSS, LiF,
and Al served as hole-, electron-injecting layer, and cathode,
respectively. For comparison, we made reference cells utilizing
the known host materials mCP (doped with FIrpic) and CBP
[doped with (PPy)2Ir(acac) and OS1] as emitting layer (Table 2
and Figure S8 in the SI).
Figure 5 and 6 show the current density–voltage–brightness
(J–V–L) characteristics, efficiency versus brightness curves,
and EL spectra of the devices. The key characteristics of the
devices are listed in Table 2. All devices displayed low turn-on
voltage at 2.5 V. The devices hosted by CzFOxa exhibited sig-
nificantly higher current density and lower driving voltage (at
1000 cd m−2 ) than that of CzFCN, which is due to more effi-
cient electron injection in EML/ETL interface. As compared to
the reference cells, the CzFCN-hosted blue device was compa-
rable to the mCP-hosted one (device B3). Both the CzFCN- and
CzFOxa-hosted devices outperformed the BCP-hosted devices
(devices Y3 and R3).
The blue emission device B1 hosted by CzFCN reveals
a maximum brightness (Lmax) of 38 200 cd m−2 at 14 V
(520 mA cm−2 ) with CIE coordinates of (0.17, 0.35). The max-
imum external quantum (ηext), current (ηc), and power efficien-
cies (ηp) were 15.1%, 31.6 cd A−1 , and 18.5 lm W−1 , respectively,
which are significantly higher than those for CzFOxa based
device B2 (9.2%, 22 cd A−1 , and 19.2 lm W−1 ). At a high bright-
ness of 1000 cd m−2 , the ηext of devices B1 and B2 are still as high
as 15.1% and 8.9%, respectively, showing very low efficiency
roll-off. The ηext of the CzFCN device was over 50% higher than
that of the CzFOxa device. This can be attributed to the higher
ET of CzFCN compared with CzFOxa, efficiently suppressing
triplet energy back transfer from the guest to the host and con-
sequently resulting in efficient blue electrophosphorescence for
the CzFCN-based device. The same trend can be observed for
devices G1 and G2 using (PPy)2Ir(acac) as dopant. The device G1
exhibited an Lmax of 150 600 cd m−2 at 14 V with CIE coordinates
of (0.35, 0.62) and attractive EL efficiencies (17.9%, 64.7cd A−1 ,
60 lm W−1 ) that are superior to those of the device G2 (14.5%,
53.2 cd A−1 , 50 lm W−1 ). At a high brightness of 1000 cd m−2 ,
the ηext of devices G1 and G2 are still as high as 17.9% and
14.4%, respectively, also showing very low efficiency roll-off.
For yellowish-green emission, we synthesized a new yel-
lowish-green emitter, m-(Tpm)2Ir(acac) by modified the lig-
ands of (Tpm)2Ir(acac). As depicted in Figure S9 (see the
SI), the EL spectrum originated from the triplet emission of
m-(Tpm)2Ir(acac) with a central electroluminescence wavelength
(λEL) of 547 nm and CIE coordinates of (0.43, 0.56), which was
red-shifted (ca. 11 nm) compared to the (Tpm)2Ir(acac)-based
device [λEL = 536 nm, CIE coordinates of (0.43, 0.56)]. It was
noted that the efficiency of the devices incorporating CzFCN
with m-(Tpm)2Ir(acac) was significantly higher than that of the
devices with (Tpm)2Ir(acac). For example, at a brightness of
1000 cd m−2 , the ηext of device with m-(Tpm)2Ir(acac) reached
17.4%, which is 1.05 times higher than the 16.5% found for
the devices with (Tpm)2Ir(acac) (see Table S2 in the SI). In
addition, we also compared CzFCN with CzFOxa as host in
m-(Tpm)2Ir(acac)-based devices. Device YG1 hosted by CzFCN
displays substantially higher efficiencies (17.4%, 60.4 cd A−1 ,
and 40 ml W−1 ) than device YG2 hosted by CzFOxa (16.2%,
53.6 cd A−1 , and 53.2 ml W−1 ). Remarkably, the device YG1 shows
rather low efficiency roll-off at a brightness of 1000 cd m−2 ,
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1000
800
600
400
200
0
(a)
(a)
CzFOxa
105
105
103
101
10-1
B2
FIrpic
1000
800
600
400
200
0
G2
YG2
Y2
(PPy)2Ir(acac)
m-(Tpm)2Ir(acac)
(Bt)2Ir(acac)
Os1
103
101
10-1
R2
0
3
6
9
12 15
0
3
6
9
12
15
Voltage(V)
Voltage(V)
60
50
40
30
20
10
0
(b)
60
20
16
12
8
(b)
20
16
12
8
50
40
30
20
10
0
4
4
0
0
100 101 102 103 104 105
Brightness (cd/m2)
100 101 102 103 104 105
Brightness (cd/m2)
(c)1.0
(c)1.0
CIE1931
CIE1931
B1 (0.17,0.35)
G1 (0.35,0.62)
YG1 (0.43,0.56)
Y1 (0.53,0.47)
R1 (0.65,0.35)
B2 (0.18,0.39)
G2 (0.35,0.62)
YG2 (0.42,0.56)
Y2 (0.52,0.48)
R2 (0.65,0.34)
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 5. a) Current density–voltage–luminance (J–V–L) characteristics.
b) External quantum (ηext) and power efficiencies (ηP) as a function of
brightness. c) EL spectra of devices for CzFCN as host.
Figure 6. a) Current density–voltage–luminance (J–V–L) characteristics.
b) External quantum (ηext) and power efficiencies (ηP) as a function of
brightness. c) EL spectra of devices for CzFOxa as host.
while the recorded ηext is still maintained as high as 17.4%.
These values are comparable to those of the best yellowish-
green phosphorescent OLEDs in the literature.[22] The emission
color and excellent performance of m-(Tpm)2Ir(acac) suggest
the good potential of employing m-(Tpm)2Ir(acac) as the emitter
for the construction of efficient WOLEDs showing evenly sepa-
rated R, G, and B colors.
Next, yellow-emitting PhOLEDs were fabricated using
(Bt)2Ir(acac) as dopant (device Y1 and Y2). Both devices exhibit
high performance. For instance, the device Y1 exhibited an Lmax
of 80 600 cd m−2 at 690 mA cm−2 (16.5 V) and very good EL
efficiencies (18%, 44.1 cd A−1 , and 34 lm W−1 ) with the CIE coor-
dinates of (0.53, 0.47) while the device Y2 exhibited an Lmax of
99 600 cd m−2 at 1020 mA cm−2 (15 V) and respectable EL effi-
ciencies (17.6%, 45.4 cd A−1 , and 47.5 lm W−1 ) with the CIE coor-
dinates of (0.52, 0.48). It is notable that, at a practical brightness
of 1000 cd m−2 , the ηext of device Y1 remained as high as 17.1%,
which is higher than that of 13.3% observed for Y2. The problem
of the rapid efficiency roll-off in the device was greatly improved
in the device hosted by CzFCN. As a result, the present phosphors
may serve as potential emitting elements for generating white
light in combination with a complementary blue phosphor.
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104
102
100
10-2
10-4
Finally, we demonstrated red-emitting PhOLEDs using the
emitter Os1 (device R1 and R2), which exhibited excellent EL
performance. For instance, the device R1 exhibited an Lmax of
26 200 cd m−2 at 650 mA cm−2 (16.5 V) and excellent EL effi-
ciencies (20%, 21.8 cd A−1 , and 20 lm W−1 ) with CIE coordi-
nates of (0.65, 0.35) while the device R2 exhibited an Lmax of
32 300 cd m−2 at 13.5 V and very good EL efficiencies (19.4%,
21.3 cd A−1 , and 22.3 lm W−1 ) with the CIE coordinates of (0.65,
0.34). The devices displayed relatively pure emission and there
are no residual emissions from the host and/or adjacent layers
in Figure 5c and 6c, indicating that the electroluminescence
is solely originated from the Os1 dopant and with completed
energy transfer from host to dopant. Considering the out-cou-
pling factor, this reaches nearly 100% internal efficiency. All
these devices from blue to red emission revealed very low turn-
on voltage (2.5 V) and excellent performances were obtained
from the same device architecture incorporated with different
phosphors. It is fascinating to mention that by using CzFCN as
the universal host, the performance of these basic-color devices
is comparable to the best one that have been reported.[23,6a–c]
(a)
Host
CzFCN
CzFOxa
104
102
100
10-2
W1
W2
0
3
6
9
12
15
Voltage(V)
25
20
(b)
20
15
10
5
16
12
8
2.7. Single-Host WOLEDs with RGB Colors
WOLEDs have been attracting particular attention because of
their potential application in solid-state lighting. Nevertheless,
how to improve device efficiency, color stability track-off, and
color-rendering index (CRI) are the critical demands of com-
mercial lighting applications. Various approaches have been
reported to improve phosphorescent WOLED performance,
which include doping of two or three phosphors that emit B–R
complementary colors or R–G–B primary colors in single- or
multi-emitting layers.[24] Among them, mutli-EMLs containing
R–G–B phosphors can improve device efficiency, color stability,
and CRI by precisely controlling exciton/charge distribution in
the devices. Here, we fabricated WOLEDs (device W1 and W2)
by distributing the R–G–B primary color emitters in multiple
regions within a single-host (CzFCN and CzFOxa), which can
reduce structural heterogeneity and facilitate charge injection
and transport between different emissive centers. The WOLEDs
have similar architecture as the aforementioned monochro-
matic devices except for EML [host: 10% FIrpic (5 nm)/host:
10% m-(Tpm)2Ir(acac) (5 nm)/host:10% OS1 (5 nm)/host: 10%
FIrpic (10 nm)]. FIrpic is a sky-blue emitter with a central emis-
sion wavelength of 473 nm while OS1 is a red emitter with a
central emission wavelength of 619 nm. In order to generate
the white spectrum with three evenly separated R, G, and B
peaks, we selected m-(Tpm)2Ir(acac) (peak at 547 nm) as the
green emitter. The emission spectra of the emitters compensate
each other so that the white spectrum generated can cover the
whole visible region.
4
0
0
100
101
102
103
104
105
Brightness (cd/m2)
Figure 7. a) Current density-voltage-luminance (J–V–L) characteristics.
b) External quantum (ηext) and power efficiencies (ηP) as a function of
brightness of white devices.
at 760 mA cm−2 (14.5 V), and excellent EL efficiencies (17.3%,
33.4 cd A−1 , and 30 lm W−1 ). Device W1’s efficiencies are higher
than those of the comparable device W2 using CzFOxa as host
(15.7%, 31.2 cd A−1 , and 28 lm W−1 ), which is probably because
device W1 exhibited a higher blue emission. In addition, the
EL spectra of devices W1 and W2 cover all wavelengths from
450 to 800 nm (Figure 8), and the color-rendering index (CRI)
is calculated to reach as high as 89. The spectra also demon-
strated highly stable chromaticity (CIEx = 0.45–0.43 and CIEy =
0.43) for device W1 and (CIEx = 0.46–0.47 and CIEy = 0.43) for
device W2 at applied voltage of 7 to 11 V. The efficient WOLEDs
with separated R, G, and B peaks and a high CRI of up to 89,
would be ideal candidates to bring OLEDs into the next genera-
tion of full-color displays and the solid-state lighting market.
Figure 7 shows the J–V–L characteristics and curves of
external quantum efficiency and power efficiency versus bright-
ness. The EL characteristics of white devices are also summa-
rized in Table 2. The turn-on voltages of devices W1 and W2
are ca. 2.5 V, almost the same as those of the monochromatic
devices, suggesting that the insertion of the mutli-EMLs did
not influence the effective charge injection and transportation.
Device W1 hosted by CzFCN achieves an Lmax of 54 300 cd m−2
3. Conclusions
Two new bipolar host molecules CzFCN and CzFOxa com-
posed of electron-rich carbazole linked through a sp3-hybridized
carbon bridge to electron-deficient cyano or oxadiazole-substi-
tuted fluorenes were synthesized and characterized. The elec-
tronic interactions between two functional constituents were
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(a)
W1
CRI CCT
7V (89.2) T=2998K
8V (89.3) T=2997K
9V (89.7) T=2985K
10V (89.6) T=3042K
11V (89.2) T=3152K
device B1
CIE (0.45-0.44, 0.43)
1.0
0.8
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device O1
device R1
619
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Wavelength (nm)
(b)
W2
CRI CCT
1.0
0.8
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0.0
CIE (0.46-0.47, 0.43)
7V (89.0) T=2852K
8V (88.6) T=2893K
9V (88.4) T=2879K
10V (88.5) T=2800K
11V (88.8) T=2773K
device B2
device O2
device R2
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400
500
600
700
800
Wavelength (nm)
Figure 8. The normalized EL spectra of white devices by hosted a) CzFCN
and b)CzFOxa at various voltages.
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triplet energy, morphological stabilities, and bipolar character
with balanced charge mobilities. These promising characters
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R–G–B colors can be realized to give outstanding efficiency and
CRI, in particular showing superior color stability. Our results
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for cost-effective fabrications of full-color PhOLED displays and
WOLED light sources can be reasonably achieved.
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Acknowledgements
The authors gratefully acknowledge financial support from the
National Science Council of Taiwan (NSC 100-2112-M-019-002-MY3,
98-2119-M-002-007-MY3, 100-2119-M-007-010-MY3).
Received: October 5, 2012
Revised: December 6, 2012
Published online: January 22, 2013
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